Circuit Note CN-0368 Devices Connected/Referenced Circuits from the Lab reference designs are engineered and Dual Channel, 1 MSPS, 12-bit, tested for quick and easy system integration to help solve todays AD7866 Simultaneous Sampling SAR ADC analog, mixed-signal, and RF design challenges. For more Integrated AMR Angle Sensor and information and/or support, visit www.analog.com/CN0368. ADA4571 Signal Conditioner Magnetoresistive Angle and Linear Position Measurements EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards The compact two-chip circuit shown in Figure 1 provides a CN-0368 Circuit Evaluation Board (EVAL-CN0368-SDPZ) contactless anisotropic magnetoresistive (AMR) measurement System Demonstration Platform (EVAL-SDP-CB1Z) solution ideal for either angle or linear position measurements. Design and Integration Files The two-chip system is capable of providing better than 0.2 Schematics, Layout Files, Bill of Materials angular accuracy over 180, and linear accuracy of 2 mil (0.002 inch) over a 0.5 inch range, depending on the size of the magnet used. The circuit is ideal for applications where high speed, accurate, noncontact angle and length measurements are critical, such as machine tool speed control, crane angle control, brushless dc motors, and other industrial or automotive applications. 5V GAIN CONTROL 5V 3.3V VDD AV DV RANGE REF SELECT DD DD VTEMP ADA4571 V TEMPERATURE SENSOR A2 V DRIVE 22nF GC BRIDGE DRIVER V AD7866 B2 10k NC A0 + VSIN EMI DRIVER G = 40 V D A A1 OUT FILTER SDP 10nF SCLK AMR BRIDGE FAULT DETECTION BIAS OSCILLATOR CS SENSORS + EMI VCOS D B DRIVER OUT G = 40 V FILTER B1 NC 10nF D A D B V AGND DGND GND CAP CAP REF 470nF 470nF 100nF Figure 1. Magnetoresistive Angle and Linear Detection System (Simplified Schematic: Decoupling and All Connections Not Shown Rev. 0 Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due Fax: 781.461.3113 2015 Analog Devices, Inc. All rights reserved. to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 13118-001CN-0368 Circuit Note Magnetoresistive (MR) Theory CIRCUIT DESCRIPTION Magnetoresistivity is the ability of a material to change the value The ADA4571 is an anisotropic magnetoresistive (AMR) sensor of its resistance when subjected to an external magnetic field. The with integrated signal conditioning amplifiers and ADC drivers, most commonly used MR sensors are based on AMR technology. as well as a temperature sensor for temperature compensation. The ADA4571 produces two analog outputs that indicate the M angular position of the surrounding magnetic field. I H The ADA4571 contains an AMR sensor and a fixed gain (G = 40 Y nominally) instrumentation amplifier. The ADA4571 delivers clean and amplified cosine and sine output signals related to the H angle of a rotating magnetic field. The output voltage range is X ratiometric to the supply voltage. Figure 2. Anisotropic Magnetoresistive Example The sensor contains two permalloy Wheatstone bridges, at a Figure 2 shows an example of the AMR effect. A current (I) relative angle of 45 to one another. A rotating magnetic field in flowing through a conductor is subject to an external magnetic the x-y sensor plane delivers two sinusoidal output signals with field (H ). The resistance of the conductor changes as a Y the double frequency of the angle () between sensor and function of the angle () between the magnetization vector (M) magnetic field direction. Within a homogeneous field in the x-y and the current flow vector (I). The magnetization vector is the plane, the output signals are independent of the physical net sum of the internal magnetic field (HX) and the applied placement in the z direction (air gap). external magnetic field (HY). The output voltage swing on the sine and cosine outputs is from The maximum resistance occurs when the magnetization vector 7% V to 93% V . There are two diagnostic bands (0% to 7% DD DD (M) is parallel to the current vector (I). The minimum resistance of V and 93% to 100% of V ), thereby providing broken DD DD occurs when the magnetization vector (M) is perpendicular to bond wire detection for all internal connections. the current vector (I). The ADA4571 is available in an 8-lead SOIC package. Effective utilization of the AMR effect requires the conductor The output impedance of the VSIN and VCOS outputs is 50 , itself to be a material insensitive to mechanical stress but and with the external 10 nF capacitors forms a 318 kHz noise filter. sensitive to magneto-restriction. For these reasons, permalloy (80% nickel, 20% iron) is the most commonly used alloy in The AD7866 is a dual channel, simultaneous sampling, 12-bit, AMR sensor manufacturing. 1 MSPS SAR ADC. The polarity of the RANGE pin determines the analog input range and output coding. If this pin is tied to a Permalloy Properties logic high when the chip select goes low, the analog input range There are two properties of permalloy strips that provide design of the next conversion is 0 V to 2 V (0 V to 5 V), leaving REF challenges when creating angular measurement systems. approximately 350 mV of headroom for the 0.35 V to 4.65 V First, permalloy has a narrow linear operating region (see signal from the ADA4571 AMR sensor. Figure 3). Only when the angle () between the magnetization Connecting the REFSEL pin low configures the ADC to use the vector (M) and current flow vector (I) becomes larger does the internal 2.5 V reference voltage. This voltage is available on the response become linear. Unfortunately, shortly after the response V pin but requires a buffer before it can be used elsewhere in REF becomes linear, the permalloy saturates. the system. The D A pin and D B pin are decoupled with CAP CAP R 470 nF capacitors to ensure proper operation of the ADC. R + R The AD7866 samples both channels of the sensor simultaneously. 0 The digital words are normally available on DOUTA and DOUTB. Each data stream consists of 1 leading 0 followed by 3 status bits and then 12 bits of conversion data. However, by holding the CS pin low for an additional 16 clock cycles, both digital words are available from one channel, D A. An SPI interface therefore OUT R 0 allows access to both channels on one data line. H Y The inputs to each of the two ADCs in the AD7866 are H 0 1.0 0.5 0 0.5 1.0 provided with a two-channel multiplexer. A Logic 0 on the A0 Figure 3. Permalloy Resistance vs. Magnetic Field input pin allows conversions on the A1 and A2 inputs, and a Secondly, permalloy is insensitive to polarity. The resistance of a Logic 1 on the A0 input pin allows conversion of the B1 and B2 permalloy strip decreases whether the angle () between the inputs. The temperature sensor output from the ADA4571 is magnetization vector (M) and the current flow vector (I) is connected to the B1 input of the AD7866 and allows software positive or negative. temperature calibration of the system. Rev. 0 Page 2 of 9 13118-002 13118-003